linux-kernel - Re: [PATCH] lib/raid6: fix abnormally high latency

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Date:   Tue, 14 Dec 2021 09:27:53 -0800
From:   Song Liu <song@...nel.org>
To:     Yajun Deng <yajun.deng@...ux.dev>
Cc:     stockhausen@...logia.de, open list <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org,
        linux-raid <linux-raid@...r.kernel.org>
Subject: Re: [PATCH] lib/raid6: fix abnormally high latency

On Mon, Dec 13, 2021 at 7:17 PM Yajun Deng <yajun.deng@...ux.dev> wrote:
>
> We found an abnormally high latency when executing modprobe raid6_pq, the
> latency is greater than 1.2s when CONFIG_PREEMPT_VOLUNTARY=y, greater than
> 67ms when CONFIG_PREEMPT=y, and greater than 16ms when CONFIG_PREEMPT_RT=y.
> This is caused by disable the preemption, this time is too long and
> unreasonable. We just need to disable migration. so used migrate_disable()/
> migrate_enable() instead of preempt_disable()/preempt_enable(). This is
> beneficial for CONFIG_PREEMPT=y or CONFIG_PREEMPT_RT=y, but no effect for
> CONFIG_PREEMPT_VOLUNTARY=y.
>
> Fixes: fe5cbc6e06c7 ("md/raid6 algorithms: delta syndrome functions")
> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>

We measure the speed of different RAID algorithms.If we don't disable
preempt, the result may be inaccurate, right? IIUC, we only disable preempt
for 16 jiffies. Why do we see 1.2 second delay?

Thanks,
Song

> ---
>  lib/raid6/algos.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
> index 6d5e5000fdd7..21611d05c34c 100644
> --- a/lib/raid6/algos.c
> +++ b/lib/raid6/algos.c
> @@ -162,7 +162,7 @@ static inline const struct raid6_calls *raid6_choose_gen(
>
>                         perf = 0;
>
> -                       preempt_disable();
> +                       migrate_disable();
>                         j0 = jiffies;
>                         while ((j1 = jiffies) == j0)
>                                 cpu_relax();
> @@ -171,7 +171,7 @@ static inline const struct raid6_calls *raid6_choose_gen(
>                                 (*algo)->gen_syndrome(disks, PAGE_SIZE, *dptrs);
>                                 perf++;
>                         }
> -                       preempt_enable();
> +                       migrate_enable();
>
>                         if (perf > bestgenperf) {
>                                 bestgenperf = perf;
> @@ -186,7 +186,7 @@ static inline const struct raid6_calls *raid6_choose_gen(
>
>                         perf = 0;
>
> -                       preempt_disable();
> +                       migrate_disable();
>                         j0 = jiffies;
>                         while ((j1 = jiffies) == j0)
>                                 cpu_relax();
> @@ -196,7 +196,7 @@ static inline const struct raid6_calls *raid6_choose_gen(
>                                                       PAGE_SIZE, *dptrs);
>                                 perf++;
>                         }
> -                       preempt_enable();
> +                       migrate_enable();
>
>                         if (best == *algo)
>                                 bestxorperf = perf;
> --
> 2.32.0
>